Various types of radar have conventionally been used to detect the relative distance and relative speed of a target. For example, radar is commonly used to detect the relative distance and relative speed of a lead-vehicle on a road. One type of radar used for this purpose is a phase monopulse radar. In a phase monopulse radar, a plurality of receiving antennas receive reflected waves from a target obtained by sending radio waves from one transmitting antenna. Because the receiving antennas have different spatial positions, even signals reflected from the same target differ in phase. The azimuthal direction of the target can then be detected by detecting this phase shift. In principle, this phase monopulse radar has an advantage in that the sending and receiving antennas need not be physically moved.
However, using a phase monopulse radar does result in the generation of ambiguities. Specifically, in the phase monopulse method, as shown in FIG. 1, the azimuthal direction .theta. of a target is obtained from the phase difference .DELTA..phi. of a signal (wavelength: .lambda.) received by two receiving antennas (assume the interval is L) in accordance with the following formula: EQU .theta.=(180/.pi.)sin.sup.-1 {(.DELTA..phi./360).multidot.(.lambda./L)}
When .DELTA..phi. satisfies -180 degrees&lt;.DELTA..phi..ltoreq.+180 degrees, a unique value for the azimuthal direction .theta. of the target can be obtained. However, when the receiving antennas have wide directional beams and the distance L between the receiving antennas is long, the value of the phase difference may be beyond this range. Therefore, a phase difference can only be detected within a practical range of -180 to +180 degrees: .DELTA..phi. and .DELTA..phi.+360 degrees, for example, cannot be discriminated.
Accordingly, when the azimuthal directions of the two targets are .theta.1 and .theta.2, respectively, and the phase difference .DELTA..phi.1 that corresponds to .theta.1 is within the above range while the phase difference .DELTA..phi.2 that corresponds to .theta.2 is beyond the range, as shown in FIG. 2, although the azimuthal direction .theta..sub.1det to be detected regarding .theta.1 is correct, the azimuthal direction .theta..sub.2det detected regarding .theta.2 will differ from the original azimuthal direction. This is what is meant by ambiguity.
In other words, when the phase shift exceeds this range, the detected azimuthal direction will differ from the true azimuthal direction. For example, if a phase monopulse radar is mounted on a vehicle and used to detect an advancing vehicle, a non-existent advancing vehicle may be incorrectly detected.
In an attempt to prevent this, a phase monopulse radar commonly only operates within a range where no ambiguity problems occur.
Another type of monopulse radar is an amplitude monopulse radar. Amplitude monopulse radar receives signals reflected from a target by a plurality of receiving antennas with different beam directions. The azimuthal direction of the target is then detected in accordance with the amplitude of the waves received by the receiving antennas. For example, if the amplitudes of the received waves received by two receiving antennas are the same, the azimuthal direction of the target is determined to be in the middle between directions of the signals, and the azimuthal direction of the target is determined from difference of the amplitude between the received waves.
Such an amplitude monopulse antenna also has a theoretical advantage in that the transmitting antenna and the receiving antennas need not be physically moved. However, operation of an amplitude monopulse antenna requires assuming that multiple targets having the same distance and speed does not exist. In other words, if waves reflected from a plurality of targets are received, these signals cannot be discriminated. Accordingly, the amplitude monopulse antenna is used only for detecting the azimuthal direction of a target in rather a narrow angle of visibility and only in the case of a single target.
It is therefore an object of the present invention to provide radar device that can accurately detect the azimuthal direction of a target using a combination of the phase and amplitude monopulse methods.